Polymers of conjugated diolefins are useful elastomeric polymers. These polymers contain a residual double bond from each diolefin monomer and this double bond provides a site for crosslinking, grafting or functionalization. Unfortunately, the double bonds are present in an excess over those generally required for these reactions. The unreacted double bonds provide sites for detrimental reactions which result in poor stability against U.V., oxidative and thermal degradation. When excellent stability is required, ethylenic unsaturation is reduced or eliminated by hydrogenation of the polymer. When both stability and reactive sites are required, it is desirable to have a controlled amount of unsaturation remaining in the polymer after hydrogenation.
Several processes to hydrogenate ethylenically unsaturated polymers are known. One class of process utilizes heterogeneous catalyst systems or carrier-supported metals. Metals known to be useful in this process include Ni, Pt, Pd, and Ru. This process can be low in activity and therefore may require high catalyst concentrations and long reaction times to obtain high levels of hydrogenation.
An improved hydrogenation process utilizes a homogeneous catalyst. Homogeneous catalysts can be prepared by combining an aluminum alkyl with a Group VIII metal carboxylate or alkoxide. The aluminum alkyl reduces the Group VIII metal and an aluminum Group VIII metal complex is formed. This hydrogenation process can be selective, hydrogenating ethylenic unsaturation but not hydrogenating aromatic unsaturation. With careful selection of hydrogenation conditions, this type of process can also be selective between different types of ethylenic unsaturation. U.S. Pat. No. 4,879,349 discloses such a selective process wherein specific process conditions result in selective hydrogenation of less sterically hindered ethylenic unsaturation while not hydrogenating a high level of the more sterically hindered ethylenic unsaturation. Faulk, in "Coordination Catalysts for the Selective Hydrogenation of Polymeric Unsaturation", Journal of Polymer Science, 9, 2617-23 (1971), also discloses a process (utilizing cobalt or nickel homogeneous catalysts) which can be selective between isoprene and butadiene unsaturation.
These processes require removal of the Group VIII metal from the hydrogenated polymer because the Group VIII metals can catalyze degradation of the hydrogenated polymer if left in the polymer. Removal is complicated and expensive due to the fine size of the colloidal catalyst suspension. It would therefore be advantageous to develop a process which would selectively hydrogenate different types of ethylenic unsaturation (i.e. butadiene and isoprene) and utilize a homogeneous catalyst which does not require a catalyst removal step.